i) Field of the Invention
This invention relates to modified elongate a fibers of a shape memory alloy, their preparation and use as actuation elements for generation of a working force.
ii) Description of Prior Art
Metal alloys are known which exhibit a shape memory effect. Such alloys exhibit a thermoelastic behavior resulting from transportation from a parent phase stable at an elevated temperature to a martensite phase at a lower temperature. If the alloy is deformed to a first particular shape while in the parent phase and is then shape adjusted while in the martensite phase to a second shape, the first shape is restored when the alloy is heated to the temperature at which transformation from the martensite phase to the parent phase occurs.
In considering the use of shape memory alloys in robotics and prosthesis, it is instructive to compare the properties of shape memory alloys with those of skeletal muscles and the ubiquitous electromagnetic actuators.
In a skeletal muscle of a mammal the tension or maximum force generated per unit cross-sectional area is a substantially constant 350 kN/m.sup.2. In comparison the maximum substainable force generated by commercial high performance linear electromagnetic motors is more than 100 times less. Thus a Bruel & Kjaer linear motor (Model B & K 4810) generates a maximum tension of 2.6 kN/m.sup.2. A short muscle having a muscle length which is the same as the muscle diameter can generate a force per unit mass of about 310 N/kg, whereas Model B & K 4810 generates only 9 N/kg. Muscle usually shortens by more than 20% in a limb, whereas Model B & K 4810 shortens by a maximum of 8%.
A skeletal muscle comprises a bundle of muscle fibers, generally in parallel relationship. More powerful muscles have more fibers, and muscles that must shorten over considerable distances have longer muscle fibers.
Robot limbs have been constructed using elongate Ni-Ti shape memory fibers. A Ni-Ti shape memory fiber having a diameter of 0.8 mm will generate over 100 MN/m.sup.2 tension and shorten by up to 10% of its length. These shape memory fibers suffer a major drawback in that the total contraction and relaxation time is unduly slow and in particular the relaxation time is unduly slow. These shape memory fibers have a total contraction and relaxation time slower than both muscle and most electromagnetic actuators.